fix(antigravity): retry transient upstream failures

Retry short-lived 5xx/capacity errors (500/502/503/504 + message
patterns) with bounded backoff capped at 15s; honor Retry-After/reset
hints and skip when wait is too long. Keep 400 non-retryable. Enable the
retry hook for 500 alongside existing 429/503.

Deduplicate sanitized Antigravity tool names before emitting the single
functionDeclarations group to avoid upstream "Tool names must be unique"
rejections.

Add Headroom size diagnostics and phantom-savings warning when reported
token delta does not shrink the outbound payload.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/providers/registry/antigravity.js

@@ -32,6 +32,9 @@ export default {
       "429": {
         attempts: 3,
       },
+      "500": {
+        attempts: 3,
+      },
       "503": {
         attempts: 3,
       },